The present invention generally relates to superconducting circuits, and more particularly to a superconducting rectifier for converting bipolar logic signals produced by a Josephson circuit to unipolar logic signals.
Superconducting circuits that operate based upon the switching of the Josephson junction are studied intensively in relation to the ultra-fast logic processors and computers. On the other hand, there is an important application of the superconducting circuits in the medical and scientific field to construct magnetometers that have an ultra-high sensitivity.
FIG. 1 shows the circuit diagram of a SQUID (superconducting quantum interference device) magnetometer proposed previously by Fujimaki et al (IEEE Transactions on Electron Devices, Vol. 35, No. 12, December 1998). The magnetometer comprises a pickup coil 1 for detecting a magnetic flux .phi..sub.x, and a superconducting path 4 is connected to the pickup coil 1 such that pickup coil 1 and the superconducting path 4 form a closed superconducting loop.
The superconducting loop 4 is coupled magnetically to another closed superconducting loop 10 that forms a SQUID sensor together with Josephson junctions J1 and J2. There, a superconducting winding 11 provided in the SQUID sensor 10 establishes a magnetic coupling M2 with a superconducting winding 2 that is formed in the loop 4, and an induction current that flows through the loop 4 in response to the interlinking of the pickup coil 1 with the unknown magnetic flux .phi..sub.x is transferred to the SQUID sensor 10. The SQUID sensor 10 is driven by a sinusoidal bias current supplied from an a.c. voltage source 12 and produces a train of output pulses with a polarity corresponding to the polarity of the unknown magnetic flux .phi..sub.x with the frequency equal to the frequency of the bias current. See FIGS. 2(A) and 2(B) wherein FIG. 2(A) shows the unknown magnetic flux .phi..sub.x schematically while FIG. 2(B) shows the output pulses produced by the SQUID sensor 10 as a result of the interlinking between the unknown magnetic flux .phi..sub.x and the pickup coil 1.
The output pulses thus produced by the SQUID sensor 10 are supplied on the one hand to an output terminal and simultaneously to a write gate 25 on the other hand via a resistor 24 and a superconducting winding 23. The write gate 25 includes a superconducting winding 22 coupled magnetically to the winding 23 as well as Josephson junctions J3 and J4, wherein the winding 22 and the Josephson junctions J3 and J4 form a closed superconducting loop. Each time the output pulse of the SQUID sensor 10 is supplied, the write gate 25 switches to a finite voltage state and stores thereby a flux quantum in a superconducting winding 26 that is connected thereto. The superconducting winding 26 in turn produces a feedback current I.sub.FB with an intensity proportional to the number of flux quanta stored in the winding 26 and supplies the same to a superconducting winding 7 that establishes a magnetic coupling M1 with the superconducting winding 3 of the loop 4. Thereby, the feedback current I.sub.FB produces a magnetic flux that counteracts the unknown flux .phi..sub.x. With increasing number of magnetic quanta stored in the winding 6, the unknown flux .phi..sub.x is gradually canceled out. When canceled out entirely, the SQUID sensor 10 produces positive and negative pulses with an equal probability. By counting the number of the positive and negative pulses and calculating the difference, one can obtain the polarity as well as the strength of the unknown magnetic flux .phi..sub.x.
In such a SQUID magnetometer whose output is digital, it should be noted that the output signal is obtained as a train of positive and negative pulses. In processing the output signal thus obtained, on the other hand, one generally needs a unipolar signal. Any logic circuits including Josephson circuits as well as semiconductor circuits assume the unipolar input pulses for the basis of the logic operation.
FIG. 3(A) is a block diagram of a Josephson up/down counter 151 that may be used for processing the output of the SQUID magnetometer of FIG. 1. The circuit is the one disclosed in the U.S. Pat. No. 4,947,118 which is incorporated herein as a reference. Referring to FIG. 3(A), the up/down counter 151 consists of a number of counting circuits 151.sub.1 -151.sub.n connected in series. A first counting circuit is supplied with a clock signal CK and an inversion thereof, CK, and outputs a binary data A.sub.1 as the LSB (least significant bit) of the counted value. A second counting circuit, on the other hand, is supplied with a binary output data X.sub.1 and an inversion thereof X.sub.1 from the first counting circuit as a carry and outputs a second binary data A.sub.2 representing the second bit of the counted value, and so on. Further, each counting circuit in the up/down counter is supplied with a reset signal RESET and thereby the binary data A.sub.1, . . . as well as the carry X.sub.1, X.sub.1, . . . are reset.
FIG. 3(B) is a circuit diagram of one of the counting circuits used in the up/down counter of FIG. 3(A). This circuit is also disclosed in the above identified reference. It will be understood that each counting circuit includes a number of AND gates represented symbolically by dots(.), and OR gates represented symbolically by plus (+). Further, there is provided a Josephson latch circuit 160 that in turn is constructed from a number of AND gates and OR gates.
FIGS. 4(A) and 4(B) are diagrams showing the construction of the OR gate and AND gate that are used in the up/down counter described previously. These circuits are described in Fujimaki et al., "Josephson Modified Variable Threshold Logic Gates for Use in Ultra-High-Speed LSI," IEEE Transactions on Electron Devices, vol. 36, no. 2, February 1989 which is incorporated herein as reference.
Referring to FIG. 4(A), the OR gate forms an asymmetric interferometer and includes therein Josephson junctions J1 and J2. In operation, the Josephson junctions J1-J3 are all in the superconducting state in the initial state. Thereby, the bias current Ig flows to the ground directly and there appears a low or zero-output at an output terminal OUT connected to the node C. The Josephson junctions J1-J3 remain in the zero-voltage state as long as there is no input current Ic. When the current flowing through the Josephson junctions J1 and J2 has exceeded a predetermined threshold as a result of increase in the input current Ic, on the other hand, the Josephson junction J1 and J2 cause a transition to the finite-voltage state. Thereby, the bias current Ig starts to flow through the Josephson junction J3 to the ground, and in response to this, the Josephson junction J3 is turned on. As a result, a finite voltage is obtained at the output terminal. More detailed analysis of the circuit can be found in the foregoing reference by Fujimaki et al.
FIG. 4(B) shows a construction of a Josephson AND gate used in the up/down counter of FIG. 3(B). Referring to FIG. 4(B), the AND gate comprises input terminals IN connected each other at a node after coupling via the illustrated OR gates described above with respect to FIG. 4(A). A Josephson junction Ja shunts the node D to the ground. In operation, the Josephson junction Ja is designed to have a threshold current of transition such that the transition occurs only when there are input currents at both input terminals IN. Thereby, the circuit produces a logic product of the input logic signals. A more complete description of the Josephson AND gate will be found in the foregoing IEEE article by Fujimaki.
From the foregoing explanation, it will be understood that a unipolar input signal is needed to activate the AND gate or OR gate that construct the up/down counter or any other logic circuits. Thus, in order to process the output of the SQUID magnetometer of FIG. 1 by a digital circuit, it is necessary to convert the bipolar pulses of the SQUID magnetometer to unipolar pulses, and for this purpose, one needs a Josephson rectifier that is operational at the liquid helium temperature environment in cooperation with the SQUID circuits. Such a rectifier circuit is also required to be capable of processing the signals having the logic amplitude of a few millivolts or less that is typical to the Josephson devices.